1. Field of the Invention
This invention relates to a window stay.
2. Discussion of the Background
The so called 4 bar window stay consists of a frame mounting plate or plates and a sash mounting plate (or plates) which are coupled by a pair of arms. Typically one arm is significantly shorter than the other arm. A pair of such stays mounted between a sash and window frame provide an effective means of controlling the opening and closing of the sash. Generally the pivot joint or bearing which couples an arm to a mounting plate is a friction joint or bearing and thus the window sash can be held at any adjusted degree of openness.
Due to the absence of sliding components and the low number of components the 4 bar stay concept provides a robust, uncomplicated and long life solution to the adjustable mounting of a window sash in a window frame. However, successful use of a 4 bar stay for large heavy side hung sashes (i.e. a casement window) which open to 90.degree. to provide good access to the outer surface of the glass for cleaning purposes is difficult. In particular if a small stay of low cost and/or compact size is used with such windows the sash tends to drop or sag. Also the operating life of the stay can prove to be inadequate.
To provide good accessibility to the outside surface of the glass for cleaning purposes, a comparatively long "short arm" is required. Such a short arm, however, has high bending moments induced in it when heavy sashes are supported. In a casement application it therefore tends to deflect downwardly allowing the sash to drop or sag. Also to provide 90.degree. opening of the sash for good cleaning accessibility 4 bar stays must have the long arm to sash plate pivot situated between the frame plate to short arm pivot and the sash plate to short arm pivot. This results in a so called "overlap" but this is usually achieved at the expense, in structural terms, of the short arm.
A further problem commonly occurring with known constructions of 4 bar stays with significant "overlap" to obtain 90.degree. opening in a stay of short overall length is that there are high internal loads in the bearings and components. Structural load analysis reveals that the bearing and arm loads can be reduced by off-setting the frame plate to the long arm bearing in a direction towards the sash outer surface (for an outwardly opening window). The effect of this is to put the loads generated in the long arm during final closing or initial opening of the sash at a more favorable angle to the sash plate and short arm. This reduces the long arm and bearing loads and hence the size of these components for a given stay life can also be reduced.
Previously known designs using an off-set frame plate to long arm bearing use either frame plates of sufficient width for the offset plus bearing width or a plate with an extension to carry the bearing (see, for example, Australian patent specification 166853). Such a design leads to unnecessary use of more expensive structural material where structural properties are not warranted.